Capillary for optical fiber connectors and method of manufacturing the same

ABSTRACT

A capillary for an optical fiber connector which is low in machining cost and a process for producing the same is provided. The capillary for an optical fiber connector of the present invention comprises a cylindrical ceramics sintered body, and having a straight-hole like narrow hole 3 with a slightly larger inner diameter than an outer diameter of the optical fiber bare line for passing the bare line of the optical fiber therethrough, and a tip surface for connection at which the narrow hole opens; and an inner surface of the narrow hole being substantially a sintered surface. Therefore, a number of steps of grinding of the inner surface of the narrow hole is markedly reduced thin the conventional capillary for an optical fiber connector so that production cost can be reduced to a low amount.

UTILIZABLE FIELD IN THE INDUSTRY

The present invention relates to a capillary for an optical fiberconnector to be used for an optical connector for transmitting anoptical signal between optical fibers wherein tip surfaces of twooptical fibers are confronted with each other.

PRIOR ART

An optical fiber connector is an article to connect optical fibers sothat optical signals are transmitted well between optical fibers byprecisely confronting tip surfaces of two optical fibers.

FIG. 17 is a sectional view showing schematic structure of arepresentative optical connector presently used. The optical connector100 connects left and right fiber optics cables 101 and 101' of thefigure.

In the fiber optics cables 101 and 101", optical fibers 102 and 102' arepassed through. The optical fibers 102 and 102' are peeled off from thefiber optics cables 101 and 101' and inserted through a narrow hole 105at the center of a capillary 104 in the optical connector 100 and fixedwith an adhesive. The capillaries 104 and 104' are set in a sleeve 106by confronting with each other to the axial direction. Both capillaries104 and 104' are connected at their tip surfaces at substantially thecenter in the sleeve 106 (capillary contacting tip surface 108, machinedto spherical or plane surface). At the contacting tip surfaces of thecapillary, optical fibers 102 and 102' should be closely contacted witheach other substantially without axial deviation or angle deviation.Said contacting tip surfaces are previously polished when manufacturinga capillary to adjust the shape of the tip surfaces, and finally, it isintegrally polished with an optical fiber on-site and then assembled.Thus, optical signals can be passed through both optical fiberssubstantially without attenuation nor reflection. Incidentally, flanges107 and 107' at the both ends of the optical connector 100 are fixed tofiber optics cables 101 and 101' by an adhesive and inserted into ahousing mating portion via a click, and the housing mating portion isconnected with a sleeve 106 (not shown) by a screw, a bionet lock, apush-on lock, etc. Therefore, a force worked between fiber optics cables101 and 101' is transferred from flanges 107 and 107' to the sleeve 106,and no force is applied to optical fibers 102.

An example of the size of respective portions of the optical connector100 is as follows.

1 Optical fiber: diameter 125 μm

2 Capillary: Outer diameter 2.5 mm, length 10.5 mm Inner diameter 126 μm

Characteristics required for a material for such an optical connectorcapillary are as follows.

1 Optical fiber is easily passed through narrow hole.

2 The material may be machined with good precision.

Particularly, precision of inner and outer diameter of narrow hole,straightness and concentricity of the narrow hole and the outerperipheral surface can be made with good precision.

3 Toughness is a certain degree or more. Even when erroneously droppedat connector assembling or connecting operation, or impact is appliedthereto, not broken.

4 In contract to the coefficient of thermal expansion 5×10⁻⁶ °C. of theoptical fiber, the coefficient of thermal expansion of the material isnot so far therefrom. If the optical fiber recedes in or projects out ofthe narrow hole of the capillary due to temperature change, it becomesloss in optical signal transmission.

As the materials for a capillary of optical fiber which is required forsuch characteristics, a ceramic sintered body has widely been used. Thereason can be considered that it is excellent in the following points ascompared with metal and plastics.

1 Since plastic deformation is hardly occurred, no deformation nor burrduring machining is occurred so that machining can be carried outprecisely. Also, when capillaries are butted, they are not deformed bybutting pressure.

2 It has good conformability with a fiber mainly composed of a glass anda fiber can be easily inserted even when clearance of an inner diameterof the capillary and an outer diameter of the fiber is 1 μm or less.

3 Difference in thermal expansion from the fiber is little and it isexcellent in heat resistance so that it is durable against change inthermal circumstance.

4 It is excellent in wear resistance so that contamination at the tipsurface due to wear powder is hardly occurred and connection failure isseldom generated at repeated connecting-and-releasing of a connector.

As for the shape or materials of a capillary for an optical fiberconnector and a process for producing the same, many numbers of patentapplications have been filed. For example, in Japanese PatentPublication No. Hei. 1-45042, a process for producing a capillary for anoptical fiber connector which contains the steps of forming an originalshape of a capillary by sintering a cylindrical molded materialcomprising alumina, etc. as a main component to which a primary hole ismade for inserting an optical fiber stock line at the center thereof,and after sintering, a wire is passed through the primary hole of thecapillary whereby polishing the primary hole by a diamond paste, etc.attached onto the surface of the wire.

In Japanese Provisional (Unexamined) Patent Publication No. Hei.1-262507, there is disclosed a capillary for an optical fiber connectorwhich comprises, in an optical fiber connector provided by a pair offerrules (capillaries) tip portions of optical fibers are each to befixed, a sleeve at which these ferrules are inserted and butted, and ameans for applying a butting pressure to these ferrules to the axialdirection; the butted tip surface of said ferrules being made convexspherical surface with curvature radius of 10 mm to 25 mm on thelongitudinal axis of said ferrules as a center. Also, there is disclosedthat the ferrule (capillary) is constituted by a zirconia ceramics.

In Japanese Provisional (Unexamined) Patent Publication No. Hei.2-304508, there is disclosed a connector for an optical fiber whichcomprises, in a connector for an optical fiber wherein an optical fiberis fitted to an axial hole provided along the center of a ceramicferrule (capillary), and optical fiber are connected by butting tipsurfaces of the ceramic ferrules for connection; said axial hole beingconstituted by a straight tube portion in which an inner diameter isconstant to the axial direction and a tapered tube portion which spreadsout toward the inserting tip surface side into which an optical fiber isinserted.

As the process for producing such a ceramics capillary, an extrusionforming method, injection molding method or powder pressing method canbe considered. However, in the case of the extrusion forming method,only a straight tube can be produced so that a tapered portion must beformed by machining. When such a post-machining is to be carried out,operation should proceed carefully so as to not leave a machining flaw.This is because when a machining flaw is generated, there is a fear ofdamaging or breaking a fiber by contacting with the portion. Therefore,the extrusion forming method is expensive in machining cost so that itcannot be said that it is suitable for the process for producing acapillary. In this point, the injection molding method and the powderpressing method are preferred since a tapered portion can be formedsimultaneously. Among these, the injection molding method which givesgood dimensional accuracy of the resulting sintered body is mostpreferred.

FIG. 18 is a sectional view of one example of the conventional mold forinjection molding. At the first mold 111, a cavity 112 for a capillaryis provided by cutting. Also, at the second mold 113, a tapered portion114 and a needle 115 are projectingly provided, and a gate 116 isprovided. The second mold 113 is matched to the first mold 111, and atthis time, a tip of the needle 115 is introduced into a concave portion111a of the first mold 111. In this state, ceramics powder containing abinder is injected into the cavity 112 with a high pressure through thegate 116.

Task to be Solved by the Invention

When the injection molded ceramic capillary was cut, it was found that anarrow hole portion formed by the needle 115 was not completely linearbut slightly curved. This is because, as shown in FIG. 18 with animaginary line, the needle 115 bent. That is, the needle 115 isextremely thin and little in flexural rigidity, and an injected compoundis not with complete uniformity spread to the circumferential surface ofthe needle 115 so that it can be estimated that the needle bent aroundthe center portion to the lengthwise direction of the needle 115.

Bending at the narrow hole portion can be allowed with a some extent butas for the portion exceeding the allowable range, it is necessary tocorrect the bending by applying "hole machining". The diameter of thenarrow hole portion is regulated to a uniform value in the finishedproduct after sintering so that when the hole machining is carried out,it is necessary to expect a margin of cutting. If it is the case, thediameter of the narrow hole portion of the molded material becomes smallwith the amount of the margin of cutting and at the same time, theneedle 115 becomes also thin. Since a thin needle becomes further thin,there occurs the vicious circle of further increasing the above bendingand deflection whereby a countermeasure thereto is required. Even in thepowder pressing method, it is difficult to apply the pressing pressureto the needle with complete uniformity so that the same problem exists.

Also, when the hole machining is applied to, an edge is likely formed atthe border of the tapered portion (103 in FIG. 17) and the narrow holeportion (105 of the same figure) so that there is a fear that the edgewill likely damage the fiber bare line to be introduced. Therefore, thehole machining is not easy and causes increase in production costs.

After all, in the conventional capillary for an optical fiber connectorand a process for preparing the same, the following problems areinvolved.

1 Narrow hole machining is essential and the cost is increased for doingthe machining.

2 An edge (machining burr) is likely formed at the cross portion of thenarrow hole and the tapered hole portion so that there are fear that afiber is broken or damaged when introducing the fiber.

An object of the present invention is to provide a capillary for anoptical fiber connector and a process for producing the same with a lowmachining cost.

Means for Solving the Task

A capillary for an optical fiber connector of the first embodimentaccording to the present invention comprises, in a capillary used for anoptical connector in which tip surfaces of two optical fiber are buttedto transmit optical signals between both optical fiber; a cylindricalceramics sintered body, having a straight-hole like narrow hole with aslightly larger inner diameter than an outer diameter of said opticalfiber bare line For passing the bare line of the optical fibertherethrough, and a tip surface for connection at which the narrow holeopens; and an inner surface of said narrow hole being substantially asintered surface.

A capillary for an optical fiber connector of the second embodimentaccording to the present invention comprises, in a capillary comprisinga cylindrical ceramics sintered body which is used for an opticalconnector in which tip surfaces of two optical fibers are butted totransmit optical signals between both optical fibers; having astraight-hole like narrow hole with a slightly larger inner diameterthan an outer diameter of said optical fiber bare line for passing thebare line of the optical fiber therethrough, a tip surface forconnection at which the narrow hole opens, a tapered hole portion whichelongates gradually tapering and positioned at an opposite side of thetip surface for connection of the narrow hole on the same axis as thenarrow hole, and a straight-hole like large hole which is connected withthe tapered hole portion on the same axis and for inserting an opticalfiber core line therethrough; and an inner surface of said narrow holebeing substantially a sintered surface.

Action

The capillaries for an optical fiber connector of the above twoembodiments according to the present invention each have a narrow holesubstantially a sintered surface so that number of steps for grinding ismarkedly reduced as compared to the conventional capillary for anoptical fiber connector. Thus, a manufacturing cost can be depressed toa low amount. Also, at the grinding of the narrow hole, an edge whichmay be formed at the cross portion of the narrow hole portion and thetapered portion, etc. is not generated. Therefore, damage of a fibercaused by the edge can be prevented. Here, the narrow hole of thecapillary is substantially a sintered surface means that remarkablenarrow hole machining (machining margin of 5 μm or more) through whichthe conventional bending is retouched is not carried out. Therefore, itis the meanings that in addition to the complete sintered surface, asurface to which a slightly retouching grinding is applied of such adegree that adhered ceramic powder is removed or a surface to whichblast machining is applied with free abrasive grains is included.

In the capillary for an optical fiber connector of the presentinvention, a molded body for a ceramics sintered body which constitutesthe aforesaid capillary may be obtained by injection molding a mixtureof ceramics powder and a binder.

This is because a capillary formed by an injection molding easily givesa sintered body having good dimensional accuracy and surface accuracythan a capillary formed by an extrusion or powder pressing.

In the capillary for an optical fiber connector of the presentinvention, it is preferred that the aforesaid optical fiber is a singlemode optical fiber, the aforesaid narrow hole diameter d_(c) is 125μm≦d_(c) ≦127 μm, and the aforesaid narrow hole length of 1.2 to 8.5 mm.As described in detail below, it is suitable for making a connectortransmission loss (connection loss) of optical signals little by makingan oblique angle (inclination) of an axis of an inserted fiber to anaxis of the capillary 0.1° or less.

Further, in order to ensure the fiber oblique angle of 0.05° or lessunder the above conditions, the above narrow hole length is preferably2.3 to 6.1 mm.

In the capillary for an optical fiber connector of the presentinvention, it is preferred that the aforesaid optical fiber is a multimode optical fiber, the aforesaid narrow hole diameter d_(c) is 126μm≦d_(c) ≦128 μm, and the aforesaid narrow hole length of 0.9 mm ormore. Further, the narrow hole length in this case is more preferably1.7 to 8.5 mm. The reason is that in the multi mode optical fiber, avalue allowable as a connection loss is larger than that of the singlemode. In general, while the maximum value of the connection lossallowable in the single mode is less than 0.5 dB, it may be less than 1dB in the multi mode. Therefore, as for the fiber oblique angle, a widetolerance is allowed in the multi mode, and it is to be ensured a valueof 0.2° or less, preferably 0.1° or less. The value of 0.9 mm or more isa value which can ensure the oblique angle of 0.2° or less, and therange of 1.7 to 8.5 mm is a value which can ensure the oblique angle of0.1° or less.

In the capillary for an optical fiber connector of the presentinvention, it is preferred that the aforesaid optical fiber is a multimode optical fiber, the aforesaid narrow hole diameter dc is 128μm≦d_(c) <130 μm, and the aforesaid narrow hole length of 1.4 mm ormore. Further, the narrow hole length in this case is more preferably2.9 to 7.5 mm. This is the same reasons as the reasons mentioned above.

In the capillary for an optical fiber connector of the presentinvention, it is preferred that the aforesaid optical fiber is a multimode optical fiber, the aforesaid narrow hole diameter d_(c) is 130μm≦d_(c) ≦132 μm, and the aforesaid narrow hole length of 2 mm or more.Further, the narrow hole length in this case is more preferably 4 to 6.1mm. This is the same reasons as the reasons mentioned above.

In the capillary for an optical fiber connector of the second embodimentaccording to the present invention, it is preferred that the sum of theaforesaid narrow hole length and the aforesaid tapered hole portionlength is 5 mm to 8.5 mm. This is because when the adhering method andan adhesive for the capillary and the optical fiber presently used areemployed, good heat-cold-impact characteristics (mentioned hereinbelow)can be obtained in the above range.

Further, the sum of the aforesaid narrow hole length and the aforesaidtapered hole portion length is more preferably 5.5 mm to 8.5 mm, andsaid length is most preferably 6 to 8.5 mm. This is because a safetyfactor against an instability factor (slight degree work failure) atconnecting work of optical fiber becomes high.

In the capillary for an optical fiber connector of the second embodimentaccording to the present invention, it is preferred that the aforesaidlarge hole length of 2 mm or more. The large hole portion is a portionwhich is to be adhered to the fiber core line, and adhesive strength ofboth can be maintained with a high degree whereby impact resistancebecomes good.

In the capillary for an optical fiber connector of the second embodimentaccording to the present invention, it is preferred that the aforesaidtapered hole portion is also a sintered surface and the border portionof the narrow hole and the tapered hole portion is a smooth surfacewithout machining burr (edge). This is because when the fiber is to beintroduced into the capillary, operation is easy and damage of the fiberbare line can be prevented.

In the capillary for an optical fiber connector according to the presentinvention, it is preferred that an oblique angle (θ_(max)) to thecapillary axis of a narrow hole opened to the tip surface for connectionis 0.1° or less in the single mode and 0.2° or less in the multi mode.This is because connection loss of optical signals at the connector canbe reduced to a low degree.

In the capillary for an optical fiber connector according to the presentinvention, it is preferred that the aforesaid inner surface roughness ofthe narrow hole is R_(a) =0.1 μm or less. This is because the fiber bareline can be introduced smoothly into the capillary, and also damage ofthe introduced fiber bare line can be prevented so that lowering instrength of the bare line can be prevented.

In the capillary for an optical fiber connector according to the presentinvention, it is preferred that the aforesaid ceramics sintered body isa zirconia series sintered body and crystal grain size at the innersurface of the aforesaid narrow hole of 0.5 μm or less. Further, theabove crystal grain size is more preferably 0.3 μm or less.

The zirconia sintered body has high toughness and hardly broken. Thosehaving a fine crystal grain size are advantageous in the point of makingthe surface roughness of the narrow hole small. Further, it isadvantageous for preventing generation of cracks at cooling aftersintering or increasing hardness of the sintered body.

In the capillary for an optical fiber connector according to the presentinvention, it is preferred that an opening angle of the aforesaidtapered hole portion is 10° to 20°. This is because the fiber bare linecan be easily introduced into the capillary and it is advantageous forpreventing damage of the fiber bare line.

In the same view, the above opening angle is more preferably 12° to 18°.Further, said angle is most preferably 14° to 16°.

The process for producing a capillary for an optical fiber connectoraccording to the present invention is a process for producing acapillary for an optical fiber connector comprising a ceramics sinteredbody provided by a straight-hole like narrow hole for inserting anoptical fiber bare line therethrough; which comprises an injectionmolding step for obtaining a molded body by subjecting a mixture(compound) of ceramics powder and a binder to an injection molding intoa mold provided by a molding pin for forming the above narrow hole, adegreasing step for removing the binder from the molded body, and asintering step of sintering the degreased molded body to obtain asintered body; and the above molding pin has a diameter d" determined bythe formula: d"=d_(c) /(s·z) from a diameter of the above narrow holed_(c), a sintering shrinkage factor s, and a shrinkage factor z atcooling solidification of the molded body.

In the process for producing a capillary for an optical fiber connectoraccording to the present invention, in order to control a discrepancy ofan angle of the narrow hole due to bending of the above molding pin atthe injection molding to a predetermined value, a narrow hole length Lmay be determined according to the following formula.

1 In the case of the single mode;

L≧1800(d_(c) -d_(f))/π and

L≦(tan 0.1°×d"⁴ E/6.79w)^(1/3) /sz

2 In the case of the multi mode;

L≧900 (d_(c) -d_(f))/π and

L≦(tan 0.2°×d"⁴ E/6.79w)^(1/3) /sz

d_(c) : Narrow hole diameter

d_(f) : Bare line diameter

d": Molding pin diameter, d"=d_(c) /sz

E: Young's modulus of the molding pin

w: Lateral load acted on the pin at an injection molding

s: Sintering shrinkage factor

z: Molding shrinkage factor

A possible maximum oblique angle θ_(max) of an axis of the fiber bareline against an axis of the capillary at the tip surface for connection(hereinafter abbreviated merely to as bare line oblique angle) isrepresented by the following formula.

    θ.sub.max =α.sub.max +βmax                (1)

α_(max) : Possible maximum oblique angle for narrow hole axis

β_(max) : Possible maximum oblique angle against narrow hole axis of thebare line axis

Here, α may be considered to be a deflection angle of the molding pin ofthe mold at an injection molding, the angle being inclination of thenarrow hole axis as such. In such a case, α_(max) is considered to be aone end supported and another end fixed beam (see FIG. 3) and shown bythe following formula. (see "Mechanical Engineering Handbook, Modified5th Edition", page 47)

    α.sub.max =tan.sup.-1 (wL".sup.3 /48EI)              (2)

w: Lateral direction distribution load acted on the molding pin at aninjection molding, it is considered to be uniform distribution load.

L": Length of narrow hole corresponding portion of the molding pin

E: Young's modulus of the molding pin constituting material

I: Geometrical moment of inertia of the molding pin

The molding pin has a circular sectional surface with a diameter d" sothat it becomes as follows.

    I=πd".sup.4 /1024                                       (3)

Formula (3) is substituted for Formula (2) as follows.

    α.sub.max =tan.sup.-1 (679wL".sup.3 /d".sup.4 E)     (4)

Here, an example of a method for obtaining the value of w (distributionload) is explained.

The maximum deflection y"_(max) of the molding pin is as follows underthe same conditions as the above deflection angle was obtained.

    y".sub.max =wL".sup.4 /184.6EI=1.77wL".sup.4 /d".sup.4 E   (5)

Also, y"_(max) is as follows from deflection y_(max) in the capillaryreal product (sintered body).

    y".sub.max =y.sub.max /(s·z)                      (6)

s: Sintering shrinkage factor

z: Linear shrinkage factor at cooling·solidification of the sinteredbody

From (5) and (6), the following is led.

    w=y".sub.max d".sup.4 E/1.77L".sup.4 =y.sub.max d".sup.4 E/(1.77L".sup.4 SZ)(7)

Here, when the data for y, s and z are obtained with respectivepredetermined conditions, w can be estimated. If so, α_(max) can beestimated from Formula (4).

On the other hand, when α_(max) is geometrically considered (see FIG.4), it is as follows.

    β.sub.max ≈tan.sup.-1 {(d.sub.c -d.sub.f)/L}≈180(d.sub.c -d.sub.f)/(πL)        (8)

d_(c) : Narrow hole diameter

d_(f) : Bare line diameter

L: Narrow hole length

From Formulae (1), (4) and (8), it is as follows.

    θ.sub.max =α.sub.max +β.sub.max =tan.sup.-1 (6.79wL".sup.3 /d".sup.4 E)+180(d.sub.c -d.sub.f)/(πL)                (9)

Here, in the case where L (L") is short, the second term of the rightside becomes predominant, while in the case where L (L") is long, thefirst term of the right side becomes predominant so that the followingcan be considered. This is because α is proportional to the cube of L".

1 In the case of the single mode;

As for the lower limit value of L:

    θ.sub.max ≈180(d.sub.c -d.sub.f)/(πL)≦0.1°Therefore, L≧1800(d.sub.c -d.sub.f)/π                                            (10)

As for the upper limit of L:

    θ.sub.max ≈tan.sup.-1 (6.79WL".sup.3 /d".sup.4 E)≦0.1° Therefore, L=L"/(sz)≦(tan 0.1°×d".sup.4 E/6.79w).sup.1/3 /(sz)         (11)

2 In the case of the multi mode;

As for the lower limit value of L:

    θ.sub.max ≈180(d.sub.c -d.sub.f)/(πL)≦0.2° Therefore, L≧900(d.sub.c -d.sub.f)/π            (12)

As for the upper limit of L:

    θ.sub.max ≈tan.sup.-1 (6.79WL".sup.3 /d".sup.4 E)≦0.2° Therefore, L=L"/(sz)≦(tan 0.2°×d".sup.4 E/6.79w).sup.1/3 /(sz)         (13)

Incidentally, θ_(max) ≦0.1° or 0.2° is one example. Based on the aboveformulae, as for specific calculation results in the case correspondingto Claims 4 to 11, they are described in the part of Examples.

In the process for producing a capillary for an optical fiber connectoraccording to the present invention, in order to estimate a discrepancyof an angle of the narrow hole due to bending of the above molding pinat the injection molding and control the discrepancy of the angle to apredetermined value or less, a viscosity of the compound for injectionmolding in a mold cavity may be controlled to a suitable value.

Also, in the process for producing a capillary for an optical fiberconnector according to the present invention, in order to estimate adiscrepancy of an angle of the narrow hole due to bending of the abovemolding pin at the injection molding and control the discrepancy of theangle to a predetermined value or less, an injection speed may becontrolled to a suitable value. Incidentally, an improvement of a moldsuch as the sum of a sprue length from a top end of the sprue to amanufacturing portion and a runner length, a ratio of the diameter ofthe runner portion and a thickness of a film gate, etc. provides aneffect directly to the viscosity and the injection molding speed,thereby mold bending is controlled so that it would be needless to saythat they are belonging to this category.

In the above estimating formula (9) of θ_(max), w (lateral load of amolding pin) is affected by a viscosity or injection speed of thecompound. Accordingly, even after determining the size of the moldingpin, a discrepancy of an angle (α_(max)) of the narrow hole can becontrolled to a regulated value or less by controlling the viscosity orinjection speed of the compound.

In the process for producing a capillary for an optical fiber connectoraccording to the present invention, the injection molding may be carriedout under the conditions satisfying the following formulae.

1 In the case of the single mode;

    tan.sup.-1 [6.79L.sup.3 s.sup.3 z.sup.3 ×exp {-A log η+B)}/(d.sub.c.sup.4 E)]+180(d.sub.c -d.sub.f)/(πL)≦0.1(14)

2 In the case of the multi mode;

    tan.sup.-1 [6.79L.sup.3 s.sup.3 z.sup.3 ×exp {-A log η+B)}/(d.sub.c.sup.4 E)]+180(d.sub.c -d.sub.f)/(πL)≦0.2(15)

A: Constant

η: Apparent viscosity value of the compound

B: Constant

It has been found that between the lateral load w acting on the moldingpin at injection molding and an apparent viscosity value η of thecompound, the following relationship exists.

    w=exp {-A log η+B)}                                    (16)

A: Constant, one example 0.46

B: Constant, one example 2.81

That is, in the element of the above one example, when the apparentviscosity value of the compound is made high, w becomes small. The factthat w becomes small means that bending of the molding pin at theinjection molding also becomes small. Accordingly, when the parametersof Formulae (14) and (15) including η are controlled, a capillary havinga small oblique angle (that is, an oblique angle of the fiber bare lineis small) of the narrow hole can be produced.

In the process for producing a capillary for an optical fiber connectoraccording to the present invention, it is preferred that the volumeratio of the ceramics powder in the above compound for injection moldingis 30 to 70%. If the volume ratio of the ceramics powder is too low,holes are likely formed in the molded body and sintered body so that asurface roughness at the inner surface of the narrow hole becomes rough.Also, a time required for degreasing of the molded body becomes long.Further, stability of the shrinkage factor becomes poor so thatdimensional accuracy of the sintered body becomes poor.

If the volume ratio of the ceramics powder is too high, the compounddoes not flow smoothly at injection molding so that surface roughening(that is, surface roughening of the sintered body) of the molded bodyoccurs. Therefore, the above-mentioned range is preferred. For stabilityof production, the volume ratio of the ceramics powder is morepreferably 40 to 60%.

Table 1 is a graph summarized evaluations of molded bodys having variousbinder compositions. The ceramics powder (ZrO₂, grain diameter 0.3 μm)is mixed by a volume ratio shown at the left column of the table. As canbe seen from the table, by using a polystyrene series, acrylic seriesand wax series binders, satisfactory results are obtained within therange of the ceramics powder volume ratio of 30 to 70%. With an acrylicseries binder, a compound using the ceramics powder volume ratio of 40or 60% is used, particularly good molded body was obtained.

                  TABLE 1                                                         ______________________________________                                        Evaluation of molded body with various compound                               compositions                                                                  (ceramics = ZrO.sub.2, grain diameter 0.3 μm)                                                    Degreasing                                                                            Shrinkage                                                                              Surface                                x   Binder     Hole   property                                                                              stability                                                                              roughening                             ______________________________________                                        20  Polystyrene                                                                              X      X       X        ◯                              series                                                                    20  Acrylic    X      X       Δ  ◯                              series                                                                    30  Polystyrene                                                                              Δ                                                                              Δ Δ  ◯                              series                                                                    30  Acrylic    Δ                                                                              Δ ◯                                                                          ◯                              series                                                                    40  Acrylic    ◯                                                                        ◯                                                                         ◯                                                                          ◯                              series                                                                    60  Acrylic    ◯                                                                        ◯                                                                         ◯                                                                          ◯                              series                                                                    70  Acrylic    ◯                                                                        ◯                                                                         ◯                                                                          Δ                                    series                                                                    70  Wax series ◯                                                                        Δ ◯                                                                          ◯                          80  Acrylic    ◯                                                                        ◯                                                                         ◯                                                                          X                                          series                                                                    80  Wax series ◯                                                                        Δ ◯                                                                          X                                      ______________________________________                                         ◯: Good, Δ: Normal, X: Poor                            

In the process for producing a capillary for an optical fiber connectoraccording to the present invention, it is preferred that the molding pinof the above injection molding mold is constituted by a high rigiditymaterial (Young's modulus of 5×10⁴ kg/mm² or more) containing WC. Thisis because bending of the molding pin at the injection molding can bemade little. In the process for producing a capillary for an opticalfiber connector according to the present invention, it is preferred thatthe above optical fiber is a multi mode optical fiber, and the diameterof the narrow hole portion of the above molding pin is 149 to 185 μm.Also, when it is the single mode one, the diameter of the narrow holeportion of the molding pin is preferably 147 to 178 μm.

When the shrinkage factor of the molded body is less than 2%, the volumeratio of the ceramics powder is 40 to 60% and a relative density of thesintered body is 100% (equal to the theoretical density), the diameterof the molding pin becomes as mentioned above. Incidentally, the narrowhole diameter of the capillary for the multi mode optical fiber is 127to 132 μm and the narrow hole diameter of the capillary for the singlemode optical fiber is 125.5 to 127 μm. Such a pin is thicker than thepin conventionally used (about 140 μm), and thus, bending of the pin atthe injection molding can be made small.

In the process for producing a capillary for an optical fiber connectoraccording to the present invention, it is preferred that a grain size ofthe above ceramics powder is 1 μm or less in average and a volume ratioof the ceramics powder is 30 to 70%. It is to ensure the characteristicsof the molded body and the sintered body. Incidentally, the averagediameter herein mentioned is a median diameter (a diameter in which acumulative distribution of 50%). As a material of the ceramics powder,there may be used Y₂ O₃ partially stabilized ZrO₂, CeO₂ partiallystabilized ZrO₂ (an application by the same applicant, filed on Mar. 10,1994, Reference No. 935513), Al₂ O₃, etc.

In the process for producing a capillary for an optical fiber connectoraccording to the present invention, it is preferred that a maincomponent of the above binder is a high molecular weight compound havingan average molecular weight of 10,000 or more. This is effective forheightening a viscosity of the compound. Examples of such a highmolecular weight compound may include an acrylic series resin, apolystyrene series resin, etc. Among these, an acrylic series resin isparticularly preferred since a shrinkage factor of the molded body islow and dimensional stability is good.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A sectional view of the capillary in accordance with one exampleof the present invention.

FIG. 2 A sectional view showing the conventional optical connector.

FIG. 3 A drawing showing deflection of a pin for a capillary injectionmolding.

FIG. 4 A drawing showing an inclination of a narrow hole of a capillaryand an optical fiber bare line.

FIG. 5 A graph showing the relationship between a length of a narrowhole portion of a capillary sintered body and the maximum deflection.FIG. 6 A graph showing the relationship between a length of a narrowhole portion of a capillary sintered body and the maximum deflectionangle.

FIG. 7 A graph showing the relationship between a length of a narrowhole of a capillary sintered body and the maximum angle oblique θ_(max)of a fiber bare line.

FIG. 8 A graph showing a variation of connection loss at an opticalfiber connecting portion prepared by using the conventional ferrule.

FIG. 9 A drawing schematically showing a capillary for an optical fiberconnector according to one example of the second embodiment of thepresent invention and an optical fiber to be introduced thereinto.

FIG. 10 A graph showing the relationship between a length of a straighttube portion with a small diameter and an adhesion strength of a fiber.

FIG. 11 A graph showing a variation of connection loss at an opticalfiber connecting portion prepared by using a ferrule of the presentexample.

FIG. 12 A graph showing a variation of connection loss at an opticalfiber connecting portion prepared by using a ferrule of Comparativeexample. FIG. 13 A sectional view of another example of a ceramicscapillary of the present invention. FIG. 14 A graph showing therelationship between a compound viscosity η (poise) and a lateral load w(kgf/mm²) applied to a molding pin. FIG. 15 A graph showing a viscosityvalue region capable of accomplishing axis discrepancy θ_(max) ≦0.2°when a narrow hole length L is changed. FIG. 16 A graph showing aviscosity value region capable of accomplishing axis discrepancy θ_(max)≦0.1° when a narrow hole length L is changed. FIG. 17 A sectional viewshowing schematic structure of a representative optical connectorpresently used. FIG. 18 A sectional view of a mold of one example of theconventional injection. FIG. 19 A graph showing the relationship betweena large hole length of a capillary and a retention strength of a fiberbare line.

EXAMPLE

In the following, Examples of the present invention and Comparativeexamples are explained. First, Comparative example in accordance with aconventional capillary having no tapered portion which is on conditionthat narrow hole machining is to be carried out is firstly explained.

By the following conditions, a sintered body for a capillary ofComparative example was prepared.

Shape: shown in FIG. 2. Characteristic feature resides in that a narrowhole 105 is long (10 mm).

Preparation method: Injection molding'sintering

Ceramics powder volume ratio: 41%

Molding pin material: Cemented carbide

Compound viscosity: 1.1×105 (poise)

Narrow hole inner diameter is as follows by expecting a cutting marginof a grinding.

Sintered body narrow hole inner diameter: 100 μm

Molded body narrow hole inner diameter: 138 μm

Molding pin outer shape: 138 μm

Incidentally, a shrinkage parameter from the molded body to the sinteredbody is set 73%.

A length of a narrow hole is as follows.

Sintered body narrow hole length: 10.5 mm

Molded body narrow hole length: 14.5 mm

The maximum deflection (y_(max)) of a narrow hole of the sintered bodyfor a capillary prepared by the above conditions was 27 μm. This dataand a molding pin diameter d"=138 μm, Young's modulus of the pin E =5.31x 104 kg/mm², total shrinkage parameter sz=0.73 and L"=14.5 mm are putin Formula (7) to obtain a lateral load acted on the molding pin atinjection molding w=1.1×10⁻⁴ kgf/mm.

Also, the above data are substituted for Formula (4) to obtain anoblique angle of the narrow hole is α_(max) =0.42° (shown in FIG. 6mentioned below with A.).

On the other hand, an oblique angle β_(max) to the narrow hole of thefiber bare line is substituted the narrow hole diameter d_(c) =132 μm(multi mode maximum diameter), the fiber bare line diameter d_(f) =125μm, L=10.5 mm for Formula (8) to obtain β_(max) =0.035°. Accordingly,angle discrepancy (an oblique angle to the capillary of the fiber bareline) of the fiber bare line θ_(max) is

    θ.sub.max =α.sub.max +β.sub.max =0.42°+0.035°=0.45°.

The value of θ_(max) is a value far greater than the limit value 0.1° ofthe single mode and the limit value 0.2° of the multi mode and ismarkedly bad and out of discussion.

Next, a capillary which is on condition that narrow hole machining isnot to be carried out is explained.

Shape example: a shape example of a capillary with a type having anarrow hole 3, a tapered hole portion 4 and a large hole 2 is shown inFIG. 1.

Material, preparation method, etc.: They are made the same as the caseof the conventional type as mentioned above except for not effecting anarrow hole machining.

Narrow hole inner diameter: The following four types were considered.

1 Single 1: Sintered body d_(c) =127 μm, Molded body d"=174 μm

2 Multi 1: Sintered body d_(c) =128 μm, Molded body d"=175 μm

3 Multi 2: Sintered body d_(c) =130 μm, Molded body d"=178 μm

4 Multi 3: Sintered body d_(c) =132 μm, Molded body d"=180 μm

FIG. 5 is a graph showing the relationship between a length of a narrowhole portion of a capillary sintered body and the maximum deflection,and according to the graph, it can be understood that when a length ofthe narrow hole portion of the capillary sintered body is small,deflection y_(max) becomes small like an exponential function.Incidentally, Comparative example is additionally shown as A. On theother hand, FIG. 6 is a graph showing the relationship between a lengthof a narrow hole portion of a capillary sintered body and the maximumdeflection angle, and according to this graph, it can be understood thatwhen a length of the narrow hole portion of the capillary sintered bodyis small, deflection angle α_(max) becomes small like an exponentialfunction.

FIG. 7 is a graph showing the relationship between a length of a narrowhole of a capillary sintered body and the maximum angle discrepancyθ_(max) of a fiber bare line. The angle discrepancy θ_(max) is

    θ.sub.max =narrow hole oblique angle α.sub.max +fiber bare line oblique angle β.sub.max

and can be calculated by Formula (9).

The angle discrepancy θ_(max) is shown in the graph of FIG. 7 when w ismade the aforesaid experimental value, d_(f) =125 μm and d_(c) =127,128, 130 or 132 μm. 1 is a representative example in the single moded_(c) =125 to 127 μm, multi 1 of 2 is a representative example at d_(c)=126 to 128 μm, multi 2 of 3 is a representative example at d_(c) =128to 130 μm, and multi 3 of 4 is a representative example at d_(c) =130 to132 μm.

The reason why the upper limit value is selected as a representativeexample, respectively, is that these are the most severe conditions.From Formula (8), the fiber bare line oblique angle β_(max) isproportioned to (d_(c) -d_(f)). Since the fiber bare line diameter d_(f)≈125 μm, the larger d_(c) is, the bigger the β_(max) is. On the otherhand, the angle discrepancy θ_(max) is shown by θ_(max) =α_(max)+β_(max) as in Formula (9) so that when dc is larger, the anglediscrepancy becomes remarkable and the conditions become severe.Incidentally, preferred relationship between the narrow hole innerdiameter d_(c) in which the upper limit value of the angle discrepancyis made 0.1° in the case of a single mode and 0.2° in the case of amulti mode, and further as a safety value, 0.05° in the case of a singlemode and 0.1° in the case of a multi mode, and the narrow hole length Lis as described above.

On the other hand, for determining the size of a hole of the capillaryfor an optical fiber connector, heat-cold-impact must also beconsidered.

In the connector of Japanese Patent Publication No. Hei 1-45042 (seeFIG. 17), the optical fiber core line 101 is supported only by a flange107, but in such a structure, it cannot be said that it can sufficientlyendure heat-cold-impact (applying heating, cooling and impactrepeatedly). Concrete example is explained in the following.

FIG. 8 is a graph showing a variation of connection loss at an opticalfiber connecting portion prepared by using the conventional ferrule, andthe transverse axis shows a number of heat-cold-impact cycles and theordinates axis shows the variation of the connection loss. Preparationconditions of the above graph are as follows.

Capillary:

Material of capillary: ZrO₂ (Yttrium partially stabilized, the ratio ofY₂ O₃ to ZrO₂ is 5.3 wt %)

Outer diameter of sintered body: 2.499±0.0005 mm

Length of sintered body: 10.5 mm

Length of small diameter straight tube portion: 10.0 mm

Diameter of small diameter straight tube portion: 125.5 to 126.0 μm

Machining method of end portion: PC abrasion (spherical surfacemachining)

Heat-cold-impact cycle:

Normal temperature→Dropped at the height of 100 mm at 75° C.→Maintainedat 75° C. for 30 minutes→Normal temperature→Dropped at the height of 100mm at -40° Maintained at -40° C. for 30 minutes→Normal temperature ismade one cycle.

A controlled value of connection loss of this class is ±0.2 dB. As for10 samples, 100 cycles test was carried out and as the results, 8samples are good, but 2 samples started fluctuation in connection lossover 20 cycles and exceeded the controlled value at 30 cycles.

That is, whereas the number of samples is small as n=10, up to 20% offailure was observed so that the conventional connector in which a fibercore line is retained only by a flange can be said to be a structurepoor in reliability by the present state adhesive method (adhesivetwo-pack type epoxy resin 353ND).

FIG. 19 is to confirm the circumstances. A tensile test was carried outby fixing only the core line with an adhesive without exposing the bareline to the ferrule. As the results, as for the product in whichfixation was carried out only by the ferrule, it had peeled off with 2kgf or so, whereas the product in which a length of the large holeportion was made 2 mm or more in the capillary for an optical fiberconnector in the second embodiment of the present invention, it showed acore line retention strength of 4 kgf or so. Thus, the length of thelarge hole portion is preferably 2 mm or more.

FIG. 9 is a drawing schematically showing a capillary for an opticalfiber connector according to one example of the second embodiment of thepresent invention and an optical fiber to be introduced thereinto.Inside of the capillary 1 for an optical fiber connector, a narrow hole3, a tapered hole portion 4 and a large hole 2 are formed. In theoptical fiber 10, a fiber bare line 12 is exposed from a fiber core line11. The fiber bare line 12 is inserted into the narrow hole 3 and thetapered hole portion 4 of the capillary 1. The fiber core line 11 isinserted into the large hole 2 of the capillary 1. At the insertedportions, an adhesive (an epoxy resin type, etc.) is adhered forfixation.

A size of the inner diameter of the capillary in Example of FIG. 9 is asfollows. Inner diameter of large hole 2: 1.0 to 1.2 mm Inner diameter ofnarrow hole 3: 125 to 127 μm (single) 126 to 132 (multi)

The length of the narrow hole 3 is made L₃, the length of the taperedhole portion 2 is made L₄, and the sum of L₃ and L₄ is made Lt.

FIG. 11 is a graph showing a variation of connection loss at an opticalfiber connecting portion prepared by using a ferrule of the presentexample, and the transverse axis shows a number of heat-cold-impactcycles and the ordinates axis shows the variation of the connectionloss. Preparation conditions of the above graph are as follows.

Material of capillary: Same as the above conventional example

Outer diameter of sintered body: 2.499±0.0005 mm

Length of sintered body: 10.5 mm

Length of small diameter straight tube portion L₃ : 6 mm

Diameter of small diameter straight tube portion: 125.5 to 128.5 μm

Length of tapered tube portion L₄ : 1 mm

Lt: 7 mm

Machining method of end portion: PC abrasion (spherical surfacemachining)

Heat-cold-impact cycle: Same as the above conventional example

In Example, after completion of 100 cycles, all 10 samples are each avariation of the connection loss of within±0.2 dB and thus they aregood. FIG. 12 is a graph showing a variation of connection loss at anoptical fiber connecting portion prepared by using a ferrule ofComparative example, and the transverse axis shows a number ofheat-cold-impact cycles and the ordinates axis shows the variation ofthe connection loss. Preparation conditions of the above graph are asfollows.

Capillary of Comparative example 1:

Material of capillary: Same as the above conventional example

Outer diameter of sintered body: 2.499±0.0005 mm

Length of sintered body: 10.5 mm

Length of small diameter straight tube portion L₃ : 3 mm

Diameter of small diameter straight tube portion: 125.5 to 126.0 μm

Length of tapered tube portion L₄ : 1 mm

Lt: 4 mm

Machining method of end portion: PC abrasion (spherical surfacemachining, R20±5 mm)

Heat-cold-impact cycle: Same as the above conventional example.

In this Comparative example, after completion of 70 cycles, among 10samples, 4 samples exceeded -0.2 dB, and in the remaining 6 samples,even one sample is not good as a tendency. Therefore, when Lt is 4 mm orless, it is not preferred in the point of lifetime.

As for heat-cold-impact cycle test, other examples are tested and theresults are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                                              Judg-                                          L.sub.t                                                                             L.sub.3                                                                              L.sub.4                                                                              Observed results                                                                         ment                                    ______________________________________                                        Example 1                                                                              5       3      2    within 0.1 dB                                                                            ◯                                                      at 50 cycles                                     Example 2                                                                              5.5     3      2.5  within 0.08 dB                                                                           ◯                                                      at 50 cycles                                     Example 3                                                                              6       3      3    within 0.05 dB                                                                           ◯                                                      at 50 cycles                                     Example 4                                                                              6       1      5    within 0.07 dB                                                                           ◯                                                      at 50 cycles                                     Example 5                                                                              7       1      6    within 0.07 dB                                                                           ◯                                                      at 50 cycles                                     Example 6                                                                              7       3      4    within 0.07 dB                                                                           ◯                                                      at 50 cycles                                     Example 7                                                                              7       6      1    within 0.06 dB                                                                           ◯                                                      at 50 cycles                                     Example 8                                                                              8.5     6      2.5  within 0.06 dB                                                                           ◯                                                      at 50 cycles                                     Comparative                                                                            4       3      1    exceeding 0.3 dB                                                                         X                                     example 1                    at 30 cycles                                     Comparative                                                                            10.5    10.1   0.4  0.3 dB or more                                                                           X                                     example 2                    at 50 cycles                                     ______________________________________                                    

In Table 2, variations of respective connection losses are examined bychanging Lt from 4 mm to 10.5 mm. In Comparative example 1 (Lt=4 mm),the loss already exceeded 0.3 dB at 30 cycles so that the judgment is X,and in Comparative example 2 (Lt=10.5 mm), the loss is 0.3 dB or more at50 cycles so that the judgment is X.

Example 1 (Lt=5 mm), Example 2 (Lt=5.5 mm), Examples 3 and 4 (Lt=6 mm),Examples 5, 6 and 7 (Lt=7 mm) and Example 8 (Lt=8.5 mm) are all good andthe judgments were O.

Further, since the judgments of any of Example 5 (L₃ =1 mm), Example 6(L₃ =3 mm) and Example 7 (L₃ =6 mm) which are all Lt=7 mm are O, it canbe found that when Lt is 5 to 8.5 mm, good results can be obtainedirrespective of L=3 mm or the value of L₄.

As the reasons why the variation of the connection loss is restrained ata low level with 5 to 8.5 mm, two reasons can be considered. First, thereason why the results are good when it is 8.5 mm or less is that thelarge hole portion for retaining the core line can be made 2 mm or more,so that the fiber core line can be certainly retained and it has animpact resistant structure.

Also, the reason why good results can be obtained when Lt is 5 mm ormore is that, as shown in FIG. 10, an adhesion strength of the fiberbare line and the ferrule is proportional to the length of the portionof which the fiber bare line and the ferrule are directly adhered to,and yet inorganic materials are bonded to each other so that thermaldeterioration is difficultly generated while an internal stress isoccurred due to the thermal cycles. Accordingly, at 4 mm, the adhesionstrength is not so large to resist to the internal stress so it involvesa problem, but good results can be obtained at 5 mm or more. Further,according to the relationship of FIG. 10, it is more safety when it ismade 5.5 mm or more, more preferably 6 mm or more.

Therefore, it can be said that the adhesion strength of the opticalfiber relates to the length Lt which is a sum of the length L₃ of thesmall diameter straight tube portion and the length L₄ of the taperedtube portion. When the Lt is in the range of 5 to 8.5 mm, morepreferably 5.5 to 8.5 mm, further preferably 6 to 8.5 mm, a ceramicscapillary which can endure the heat-cold-impact test can be provided.Also, by making the tapered tube portion slightly long, the capillaryhas a structure which can endure heat-cold-impact even when the smalldiameter straight tube portion is shortened, has a little deformation inthe tube at the injection molding step and a lifetime of the molding pinbecomes long.

FIG. 13 is a sectional view of another example of a ceramics capillaryof the present invention. In this ceramics capillary 21, a smalldiameter straight tube portion 23, a tapered tube portion 23 (a taperedtube portion 24 comprises a first tapered tube portion 25, straight tubeportion 26 and a second tapered tube portion 27.), and a large diameterstraight tube portion 22 are successively provided, and the tapered tubeportion 23 and the tapered tube portion 24 contribute to improveadhesion strength, and yet the small diameter straight tube portion 23can be shortened sufficiently.

That is, the tapered tube portion 24 may be a tapered tube portioncontaining a small diameter straight tube portion 23 and a largediameter straight tube portion 22 at a tapered portion (in this example,25 and 27), and there is no problem to contain one or more straight tubeportion(s) therein.

Next, by paying attention to the viscosity of the compound for injectionmolding, the process for preparing a capillary of the present inventionwhich controls the preparation parameter of a capillary for an opticalfiber connector is explained.

An angle discrepancy θ_(max) of a fiber core line is given by theaforesaid Formula (9) as shown below.

    θ.sub.max =α.sub.max +β.sub.max =tan.sup.-1 (6.79wL".sup.3 /d".sup.4 E)+180(d.sub.c -d.sub.f)/(πL)                (9)

For this Formula, the above Formula (16) which is an experimentalformula showing the relationship of w and η is substituted.

    w=exp {-(A log η+B)}                                   (16)

The result is Formula (14) in the single mode and Formula (15) in themulti mode.

    tan.sup.-1 [6.79L.sup.3 s.sup.3 z.sup.3 ×exp {-(A log η+B)}/d.sub.c.sup.4 E]+180(d.sub.c -d.sub.f)/πL≦0.1(14)

    tan.sup.-1 [6.79L.sup.3 s.sup.3 z.sup.3 ×exp {-(A log η+B)}/d.sub.c.sup.4 E]+180(d.sub.c -d.sub.f)/πL≦0.2(15)

A: Constant

η: Apparent viscosity value of the compound

B: Constant

In these Formulae 14 and 15, as a desirable θ, it is set ≦0.1° for thesingle mode and ≦0.2° for the multi mode.

The relationship of these Formulae is specifically observed.

In order to specifically ascertain the above experimental formula of w,η (apparent viscosity value of the compound) was obtained according tothe following manner in the examples of the present invention. Thetemperature of the compound is made 30° C. lower than the temperature ofthe compound in a cylinder of the injection molding machine,specifically 120° C. And a load 20 (kgf/mm²) is applied to the compoundto pass through a capillary with a diameter of 1 mm×length of 1 mm. Apassed amount of the compound per a unit time at the procedure ismeasured. From the measured value, η was calculated by Newtonian fluidapproximation.

Next, the relationship between the η and w was obtained by an experimentwith the following conditions. Used ceramics powder: material ZrO₂,average grain diameter 0.3 μm, volume ratio 41%.

Binder: Acrylic type resin, average molecular weight 20,000

Molding pin diameter: 0.180 mm

Compound temperature: Cylinder portion 150° C., Cavity portion estimated120° C.

Injection speed: 5%

FIG. 14 is a graph showing the relationship between a compound viscosityη (poise) and a lateral load w (kgf/mm²) applied to a molding pin. Inthis case, the constants in the above Formula (13) were A=0.46 andB=2.81.

    w=exp {-(0.46 log η+2.81)}                             (15)

By using the above formulae, a desired compound viscosity value isobtained.

In the case of the multi mode

Shrinkage factor of the molded body z=0.99,

Young's modulus of the molding pin E=5.31×10⁴ kg/mm²,

L=10.5 mm,

d_(c) =0.1285 mm,

d_(f) =0.125 mm,

These values are substituted for the left side, the first term ofFormula (12), and by changing the ceramics volume ratio from 0.3 to 0.7and η from 1×10⁴ to 1×10⁶ poise to obtain α_(max). The results are shownin Table 3.

                  TABLE 3                                                         ______________________________________                                        Ceramics    Compound vis-                                                     volume ratio x                                                                            cosity value η                                                                           α.sub.max                                                                      Judgment                                    ______________________________________                                        0.3         1 × 10.sup.4                                                                           0.45°                                                                         X                                           0.3         1 × 10.sup.5                                                                           0.19°                                                                         X                                           0.3         1.2 × 10.sup.5                                                                         0.16°                                                                         ◯                               0.3         1 × 10.sup.6                                                                           0.05°                                                                         ◯                               0.4         1 × 10.sup.4                                                                           0.49°                                                                         X                                           0.4         1 × 10.sup.5                                                                           0.23°                                                                         X                                           0.4         1.5 × 10.sup.5                                                                         0.16°                                                                         ◯                               0.4         1 × 10.sup.6                                                                           0.06°                                                                         ◯                               0.5         1 × 10.sup.5                                                                           0.24°                                                                         X                                           0.5         1.7 × 10.sup.5                                                                         0.16°                                                                         ◯                               0.5         1 × 10.sup.6                                                                           0.06°                                                                         ◯                               0.6         1 × 10.sup.5                                                                           0.26°                                                                         X                                           0.6         1.9 × 10.sup.5                                                                         0.16°                                                                         ◯                               0.6         1 × 10.sup.6                                                                           0.07°                                                                         ◯                               0.7         1 × 10.sup.5                                                                           0.27°                                                                         X                                           0.7         2.2 × 10.sup.5                                                                         0.16°                                                                         ◯                               0.7         1 × 10.sup.6                                                                           0.07°                                                                         ◯                               ______________________________________                                    

From this Table 3, when the compound viscosity value η is made η≧2.2×10⁵poise, it can be understood that α_(max) can take the desired valueα_(max) ≦0.16° (because β_(max) is 0.04° when L=10.5 mm) at L=10.5 mmeven when the ceramics volume ratio x may take any value within0.3≦×≦0.7. Also, as for the more preferred ceramics volume ratio of0.4≦×≦0.6 in view of injection moldability, the width of the compoundviscosity value η which gives x_(max) ≦0.16 is spread and it may beη≧1.9×10⁵ poise.

In the case of the single mode

By making d_(c) =0.127 mm, d_(f) =0.125 mm, L=10.5 mm and others are thesame as in the case of the multi mode, calculation was carried out. Theresults are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Ceramics    Compound vis-                                                     volume ratio x                                                                            cosity value η                                                                          α.sub.max                                                                       Judgment                                    ______________________________________                                        0.3         1 × 10.sup.5                                                                          0.17°                                                                          X                                           0.3         4.0 × 10.sup.5                                                                        0.089°                                                                         ◯                               0.4         1 × 10.sup.5                                                                          0.19°                                                                          X                                           0.4         5.0 × 10.sup.5                                                                        0.089°                                                                         ◯                               0.5         1 × 10.sup.5                                                                          0.20°                                                                          X                                           0.5         6.0 × 10.sup.5                                                                        0.089°                                                                         ◯                               0.6         1 × 10.sup.5                                                                          0.21°                                                                          X                                           0.6         6.6 × 10.sup.5                                                                        0.089°                                                                         ◯                               0.7         1 × 10.sup.5                                                                          0.21°                                                                          X                                           0.7         7.2 × 10.sup.5                                                                        0.089°                                                                         ◯                               ______________________________________                                    

From this Table 4, when the compound viscosity value η is made η≧7.2×10⁵poise, it can be understood that α_(max) can take the desired valueα_(max) ≦0.089° (because β_(max) is 0.011° when L =10 mm) at L=10.5 mmeven when the ceramics volume ratio x may take any value within0.3≦×≦0.7. Also, as for the more preferred ceramics volume ratio of0.4≦×≦0.6 in view of injection moldability, the width of the compoundviscosity value η which gives x_(max) ≦0.089 is spread and it may beη≧1.9×10⁵ poise.

As mentioned above, it has been shown that when the compound viscosityvalue is changed in place of changing the value of the narrow holelength L, bending at forming can be depressed whereby a ferrule whichcan previously prevent discrepancy of axis at butting in the opticalfiber connector.

Next, it will be now shown about the case of justifying the narrow holelength L and the compound viscosity value η simultaneously.

In the case of the multi mode It was made d_(c) =0.1285 mm, and d_(f)=0.125 min.

Table 5 shows a range of the viscosity value which accomplishes the axisdiscrepancy θ_(max) ≦0.2° when the narrow hole length L is changed. FIG.15 is to show the fact with a region on the graph.

                  TABLE 5                                                         ______________________________________                                        Narrow Maximum   Allowable Compound viscosity                                 hole   oblique   deflection                                                                              allowable value η                              length L                                                                             angle β.sub.max                                                                    angle α.sub.max                                                                   poise                                              mm     °  °  0.4 ≦ x ≦ 0.6                                                            0.3 ≦ x ≦ 0.7               ______________________________________                                        10.5   0.04      0.16      1.9 × 10.sup.5 or                                                                2.2 × 10.sup.5 or                                              more     more                                      7      0.06      0.14      1.5 × 10.sup.4 or                                                                1.7 × 10.sup.4 or                                              more     more                                      4      0.10      0.10      8.2 × 10.sup.2 or                                                                9.3 × 10.sup.2 or                                              more     more                                      3      0.13      0.07      2.7 × 10.sup.2 or                                                                3.1 × 10.sup.2 or                                              more     more                                      2      0.20      --        --       --                                        ______________________________________                                    

From the results, the following can be understood. First, when L is madesmall, β_(max) becomes large. And L is L<2 true, β_(max) becomesβ_(max) >0.2°. Since θ_(max) =α_(max) +β_(max) and α_(max) ≧0, in thiscase, θ_(max) becomes θ_(max) >0.2° to any viscosity values whereby axisdiscrepancy cannot be depressed to the suitable value.

At L>2 mm, when L is made small, β_(max) becomes large so that α_(max)allowable value becomes severe. However, from Formula (13), α_(max) isproportional to cube of L so that an allowable range of the viscosityvalue η in Formula (15) becomes rather broad.

Therefore, in L>2 mm, as L is little, designation of the viscosityconditions becomes easy. In general, the viscosity of the compound whenentering into a cavity is suitably 1×10⁴ to 10⁵ poise or so, and whenthe value exceed the range, Short-Short, etc. are likely caused andmolding becomes extremely difficult. Also, the viscosity is too low,bubble, etc. is easily caught up at molding. Selection conditions of abinder are also limited. For such a reason, when injection molding iscarried out at 2 L≦9.6 mm which can designate the viscosity conditionsto 1×10⁴ to 10⁵ poise, a capillary with less bending can be produced.

In the case of the single mode

d_(c) =0.126 mm

d_(f) =0.125 mm

Table 6 shows a range of the viscosity value which accomplishes the axisdiscrepancy θ_(max) ≦0.1° when the narrow hole length L is changed. FIG.16 is to show the fact with a region on the graph.

                  TABLE 6                                                         ______________________________________                                        Viscosity conditions satisfying the single mode angle                         discrepancy regulated value                                                   Narrow hole            Compound viscosity                                     length L    α.sub.max (Regu-                                                                   value η                                            mm          lated value)                                                                             poise                                                  ______________________________________                                        10.5        0.089°                                                                            7.2 × 10.sup.5 or more                           9           0.087°                                                                            3.5 × 10.sup.5 or more                           7.5         0.085°                                                                            1.2 × 10.sup.5 or more                           6           0.080°                                                                            3.2 × 10.sup.4 or more                           4.5         0.074°                                                                            1.4 × 10.sup.4 or more                           3           0.061°                                                                            1.5 × 10.sup.3 or more                           1.5         0.023°                                                                            1.7 × 10.sup.2 or more                           ______________________________________                                    

From the results, the following can be understood. First, as in the caseof the multi mode, when L is made small, β_(max) becomes large. And L isL<1.1 mm, θ_(max) becomes β_(max) >0.1°. Thus, in L<1.1 mm, θ_(max)becomes θ_(max) >0.1° to any optional viscosity whereby axis discrepancycannot be depressed to the suitable value.

At L>1.1 mm, as shown FIG. 16, when L is made small, designation of theviscosity conditions becomes easy. By the same reason as in the case ofthe multi mode, it is considered that the viscosity setting conditionsare suitable 104 to 105 poise, and for the purpose of these, it ispreferred to made 1.1<L≦7.2 mm.

Next, viscosity values with compositions in various binder systems, aninjection temperature (a cylinder temperature), said temperature -30° C.(estimated cavity temperature) are shown in Table 7.

                                      TABLE 7                                     __________________________________________________________________________         Organic additives constitu-                                                                      Ceramic                                                                            Injec-                                           Ceramics                                                                           tional ratio (%)   particle                                                                           tion                                             volume                                                                             Acrylic                                                                            Wax Styrene                                                                            Low  diameter                                                                           tempera-                                                                           Viscosity                                   ratio x                                                                            series                                                                             series                                                                            series                                                                             molecule                                                                           (μm)                                                                            ture (°C.)                                                                  (poise)                                     __________________________________________________________________________    0.47 --   64.6                                                                              22.8 12.6 0.95 180  5 × 10.sup.3                          0.60 --   86.9                                                                              --   13.1 0.95 110  2 × 10.sup.3                          0.56 89.7 --  --   10.3 0.95 140  1 × 10.sup.4                          0.40 93.7 --  --   6.3  0.95 110  4 × 10.sup.5                          0.30 95.3 --  --   4.7  0.95 100  4 × 10.sup.6                          0.53 66.6 14.6                                                                              12.5 6.3  0.3  150  1 × 10.sup.4                          0.53 72.9 25.0                                                                              --   2.1  0.3  150  6 × 10.sup.4                          0.40 65.9 31.8                                                                              --   2.3  0.3  100  2 × 10.sup.4                          0.53 89.7 --  --   10.3 0.3  160  1 × 10.sup.5                          0.40 93.7 --  --   6.6  0.3  120  4 × 10.sup.6                          0.40 66.6 31.2                                                                              --   2.1  0.07 150  1 × 10.sup.5                          0.35 89.1 --  --   8.9  0.07 160  1 × 10.sup.6                          0.30 93.7 --  --   6.3  0.07 140  5 × 10.sup.6                          __________________________________________________________________________

From Table 7, the following can be understood.

1 When the grain size of the ceramics is made small, viscosity becomesextremely high so that the volume ratio of the ceramics must be small.When the viscosity becomes 10⁵ poise or more, there is a fear ofgenerating Short-Short, or the like, and when it exceeds 10⁶ poise, ifthe molding conditions are not extremely severely adopted, Short-Shorthad actually occurred.

2 For heightening the viscosity value of the compound, it is effectiveto add a high molecular weight material which exceeds an averagemolecular weight of 10,000 such as an acrylic series resin with a muchamount. Particularly when an acrylic resin is used, there is a tendencythat the molding shrinkage factor becomes small as 0.6% or less so thatit is more preferred than the other high molecular weight materialexceeding an average molecular weight of 10,000 in view of dimensionalaccuracy.

3 In the case of the single mode, when L=1.5 to 3 mm, any of thecompounds shown in Table 7 is used, there is no effect on an anglediscrepancy of the fiber bare line. When L=3 mm or more, it is preferredto use a compound comprising an acrylic series resin as a maincomponent. Further, when L=10.5 mm, it is necessary to η≧7.2×10⁵ poiseso that even when ceramics powder having an average grain size of 0.07μm is used, it is necessary to make the acrylic series resin 80% or morein the binder organic additives.

In the case of the multi mode, when L=2 to 4 mm, any of the compoundsshown in Table 7 is used, there is no effect on an angle discrepancy ofthe fiber bare line. When L=4 mm or more, it is preferred to use acompound comprising an acrylic series resin as a main component.Further, when L=10.5 mm, it is necessary to η≧2×10⁵ poise so that evenwhen ceramics powder having an average grain size of 0.07 μm is used, itis necessary to take the acrylic series resin 70% or more in the binderorganic additives.

An example of representative manufacturing conditions is described.

Capillary shape: Single mode, narrow hole length 3 mm, tapered holeportion 6 mm, whole length 10.5 mm

Formulation:

5.3% by weight of Y₂ O₃ -added ZrO₂ powder (average grain diameter 0.07μm) obtained by the hydrolysis method and a binder were formulated so asto become the volume ratio of the ceramics powder of 40 to 50% byvolume. For the binder, an acrylic series resin is mainly used as amolding agent. Also, DBP as a plasticizer and a wax as a lubricatingagent are added.

Mixing and kneading:

The above-mentioned composition was subjected to the steps of mixing at100° to 150° C., and kneading at 60° to 80° C. were carried out 1 to 5times to prepare a kneading material.

Granulation:

The above-mentioned kneading material was granulated by using apelletizer.

Molding:

The above-mentioned granulated material was subjected to injectionmolding at a cylinder temperature of 120° to 160° C., a mold settingtemperature of 20° to 40° C., an injection pressure of 900 to 1800kg/cm², a dwell pressure application pressure of 180 to 800 kg/cm², adwell time of 0.5 to 5 seconds. Other molding conditions are, dependingon the kind or the shape of the ceramics, selected from conditionswherein defects such as crack, sink mark, Short-Short, weld, flow mark,etc. are not generated to carry out molding. At that time, it ispreferred that the sum of the spure length from the spure to the productportion and the runner length is 50 to 100 mm and the ratio of thediameter of the runner portion and the thickness of the film gate is 1.5to 5.

Degreasing:

Degreasing is carried out by using a pressure degreasing furnace.Temperature elevation at degreasing is 180° to 230° C. or so and forgradually decomposing a low molecular weight component(s), thetemperature is gradually elevated. Further, at the neighbor of 250° C.at which changes are most remarkable by the TG-DTA curve, thetemperature is retained to avoid generation of cracks, craze, voids,etc. The time schedule of the respective steps is different depending onthe shape, and in a test piece having a large size, the temperature iselevated for a longer time than that of the capillary.

Sintering:

Under atmospheric pressure, sintering is carried out at 1300° to 1500°C. for 2 hours. The temperature elevating rate is 50° to 200° C./hr, andwhen reducing the temperature, it is reduced with 50° to 200° C./hr upto 800° C. and then cooled in a furnace.

As clearly seen from the above explanation, the present inventionexhibits the following effects.

1 A capillary for an optical fiber connector which is a low machiningcost can be provided.

2 A capillary for an optical fiber connector which is easy for insertingan optical fiber and having no fear of damaging the fiber when insertioncan be provided.

3 A capillary having good heat-cold-impact characteristics and impactresistant characteristics can be provided.

We claim:
 1. A capillary for an optical fiber connector which comprises,in a capillary used for an optical connector in which tip surfaces oftwo optical fibers are butted to transmit optical signals between bothoptical fibers;a cylindrical ceramics sintered body, having astraight-hole like narrow hole with a slightly larger inner diameterthan an outer diameter of said optical fiber bare line for passing thebare line of the optical fiber therethrough, and a tip surface forconnection at which the narrow hole opens; and an inner surface of saidnarrow hole being substantially a sintered surface.
 2. A capillary foran optical fiber connector which comprises, in a capillary comprising acylindrical ceramics sintered body which is used for an opticalconnector in which tip surfaces of two optical fibers are butted totransmit optical signals between both optical fibers;having astraight-hole like narrow hole with a slightly larger inner diameterthan an outer diameter of said optical fiber bare line for passing thebare line of the optical fiber therethrough, a tip surface forconnection at which the narrow hole opens, a tapered hole portion whichelongates gradually tapering and positioned at an opposite side of thetip surface for connection of the narrow hole on the same axis as thenarrow hole, and a straight-hole like large hole which is connected withthe tapered hole portion on the same axis and for inserting an opticalfiber core line therethrough; and an inner surface of said narrow holebeing substantially a sintered surface.
 3. A process for producing acapillary for an optical fiber connector comprising a ceramics sinteredbody provided by a straight-hole like narrow hole for inserting anoptical fiber bare line therethrough;which comprises an injectionmolding step for obtaining a molded body by subjecting a mixture(compound) of ceramics powder and a binder to an injection molding intoa mold provided by a molding pin for forming the above narrow hole, adegreasing step for removing the binder from the molded body, and asintering step of sintering the degreased molded body to obtain asintered body; and the above molding pin has a diameter d" determined bythe formula:

    d"=d.sub.c /(s·z)

from a diameter of the above narrow hole d_(c), a sintering shrinkagefactor s, and a shrinkage factor z at cooling solidification of themolded body.